The terahertz wave is of great importance in the application in fields such as electronics, communication, life science, national defense, aerospace, medical industry, and so forth. The terahertz functional devices are the essential parts in a terahertz system. In a terahertz high-speed imaging and communication system, a high-performance modulator plays an importance role. Nowadays, the development and application of electrically driven terahertz modulator with high modulation speed and large modulation depth have been hindered due to its absence in global industry.
The technical solution of the existing solid terahertz modulator mainly refers to the mechanism of non-resonance absorption of terahertz wave caused by the changes of Drude conductivity based on single-electron behavior. The modulator can be achieved mainly by the following materials: semiconductor two-dimensional electron gas, semiconductor hybrid metamaterial, graphene, and so forth.
Regarding the modulator achieved by two-dimensional electron gas, the conductivity thereof can be changed by electrically controlling the concentration of the two-dimensional electron gas through a gate, so as to change a transmission intensity of an incident terahertz radiation. The above method can be performed at an ambient temperature, while the maximum modulation depth thereof is only 3%, which can be hardly promoted in actual application. Such type of modulator does not utilize characteristics of plasma wave. The principle is that the transmission intensity of terahertz wave is related to the conductivity of the two-dimensional electron gas.
Regarding the modulator achieved by semiconductor hybrid metamaterial, the metamaterial is an artificial media constituted by structural units (“atoms”) smaller than wavelengths of excitation electromagnetic wave and having electromagnetic resonance response. When manufacturing the semiconductor hybrid metamaterial, adjustments can be made in terms of the geometric design and the electromagnetic structural parameters of the metamaterial in order to change the resonant characteristics thereof. Based on such principle, an effective manipulation of the terahertz radiation can be achieved. A Schottky diode structure is formed by manufacturing metamaterial on a doped semiconductor epitaxial layer, and the resonance intensity can be changed by tuning the carrier concentration of a semiconductor substrate layer near a gap of a split-ring resonator (SRR) structural unit through voltage, thus the transmission intensity of the terahertz wave at the resonance frequency can be controlled electrically. The above method can achieve a modulation depth of 50% and a modulation speed of 2 MHz at an ambient temperature. In another hybrid metamaterial structure, the high electron mobility transistor (HEMT) is integrated at the gap of the SRR, and the capacitance of the SRR can be changed by tuning the electron concentration in the channel through the gate, in order to adjust the resonance intensity of the SRR. The modulation depth of the device can reach 33%, and the highest modulation speed can reach 10 MHz. In the terahertz band, the existing patents of modulator are mainly achieved by using metamaterial, such as the US patent with the title of “Active Terahertz Metamaterial Device” filed by Houtong Chen et al in 2009.
Regarding the modulator achieved by graphene, in the terahertz band, the intraband transition of electrons in the graphene plays the major role, while a modulation depth of 15% and a modulation frequency of 20 kHz can be achieved at an ambient temperature by using a single-layer graphene having a large area.
The major defects of the prior art mainly include:                in the prior art, the modulation depth is generally low, and may only reach as high as 50%, which means that the energy dissipation of the above mechanisms are not very effective; in addition, high speed modulation is one of the most important performance metrics of the modulator, while the modulation speed of the prior art is not high (the highest modulation speed is 10 MHz).        
The underlying causes of these defects mainly include: a) the carrier layers of the two-dimensional electron gas and the graphene are very thin, the electromagnetic wave interacts with the carrier for a short period of time, and if a strong coupling is not achieved, the modulation efficiency will not be high; b) the dissipation of a Drude conductivity model originates from single free carrier scattering by external environment (phonons, impurities and defects, etc.), and the electromagnetic wave is coupled with single-particle, hence such dissipation mechanism of the terahertz wave is not very effective; c) the modulation speed of a device with a large active area is limited by a parasitic capacitance and resistance, such as the depletion layer capacitance and resistance formed by applying gate voltage in a semiconductor hybrid metamaterial, of the device.
So far the relevant papers and reports on achieving terahertz modulator using low-dimension electron plasma wave is not yet published.